While researching the answer to "How was Germany's border decided?", I didn't manage to find any documents which would precisely describe the borders between Germany and Poland between the points 53° 16' 28.19" N 14° 26' 38.96" E (near Mescherin and Gryfino) and 53° 55' 45.45" N 14° 13' 40.78" E (near Świnoujście and Heringsdorf). Also, I didn't find any documents which would describe the precise sea border north of the second point before 1989-05-22.

The Yalta (February 1945) and Potsdam (August 1945) conferences' protocols refer to the Oder-Neisse line as the western border of future Poland, even though the Soviets gave the authority over most of Stettin/Szczecin (which lies west of this line) and surrounding areas to the provisional Polish government on 1945-07-05 already. The same happened with Swinemünde/Świnoujście on 1945-10-06, shortly after the end of the Potsdam conference.

The Treaty of Zgorzelec of 1950-07-06 between East Germany and Poland only re-affirmed the Oder-Neisse line as the border and left Świnoujście to be Polish (without clearly defining what this border was), but it left out the question about the parts west of the Oder which were still administrated by Poland. Another part of the treaty called for creating a joint agency to decide on the exact points of the border, whose publications would likely be the normative source of the border course, but I wasn't able to locate any works by said agency to verify it yet.

There was another treaty between East Germany and Poland, signed on 1951-01-27 in Frankfurt on the Oder by the East German foreign secretary Georg Dertinger and the leader of the Polish foreign ministry Stanislaw Skrzeszewski; this could be another normative document about the border course, but I wasn't able to locate it either.

The Treaty of Warsaw of 1970-12-07 between West Germany and Poland only refereed to the border "... as defined in the previous treaties", specifically referring to the one from 1951.

Another treaty, signed on 1989-05-22 in Berlin (see German version) between East Germany and Poland re-defines the sea border north of the island of Usedom/Uznam, but doesn't include the previous course.

Further treaties between unified Germany and Poland only re-affirm the status quo, but don't contain a formal definition of if, referring to earlier treaties.

1 Answer
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The answer is apparently: this border isn't defined anywhere. As you correctly noted, the result of the Potsdam Conference was the Oder-Neiße-line as Poland's western border, without any exception for Stettin. The sources that I looked at agree that the Soviet Union violated that agreement and gave the area around Stettin to Poland in July 1945. It is unclear what prompted this decision, I couldn't find anything in the Russian sources. Most likely this was simply additional compensation for the territories Poland lost to the Soviet Union. Neither could I find any reaction from the other allies - it seems that they didn't consider the matter important enough.

There is lots on this in the German sources however. In particular, there is a documented parliamentary debate in West Germany following the Treaty of Warsaw in 1970. I will try to give a rough translation:

Member of parliament Rollmann asks:

1) Which west border Poland's did the German government accept in the
Treaty of Warsaw, the Oder-Neiße-line as defined by the Potsdam
Conference or the additionally changed to German disadvantage
Pseudo-Oder-Neiße-line?

2) When the German government was discussing the Warsaw Treaty, did it
overlook that contradictory to the decision of the Potsdam
conference a large area around Stettin to the west of Oder was put
under Polish administration?

Answer from state secretary Moersch:

1) The German government didn't define any borders in the Warsaw
Treaty, it didn't have any possibility to do that. It merely accepted
that the already existing border is the western border of Poland.
Changing the current border line was not possible.

2) The German government didn't overlook it. However, we had to base
our decision on two undeniable facts: the decisions of the Postdam
Conference and the existing border. Germany cannot change anything on
either these decisions or the existing border.

Poland was worried that Germany would again discuss this border after reunification. So the Treaty on the Final Settlement with Respect to Germany from 1990-09-12 once again states explicitly that Germany recognizes its existing border to Poland (German text of the treaty, see Artikel 1, (2)). But even this treaty doesn't describe the border - it simply refers to the existing border because at that point this border was simply a fact.

We're talking about Germany here. Every cm² has to be accounted for, or else! ;) In all seriousness, my further searches point to the 1951 treaty (see also the footnote about it in Khan's "Die Deutschen Staatsgrenzen", page 332 and the treaty itself), but I didn't find anything conclusive yet.
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Martin SojkaOct 27 '11 at 14:28

@MartinSojka: That's the Treaty of Zgorzelec you found already. Art. 5 talks about an additional document describing the exact border line, that document was created in 1951 and Khan refers to it. So maybe it does indeed describe the border with Stettin belonging to Poland - now one would need to find the text...
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Wladimir PalantOct 27 '11 at 14:37

Accepting the answer for now; I think you're right, but I'll check with the German "Auswärtiger Amt" too. Would be nice to have this data online somewhere ...
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Martin SojkaOct 27 '11 at 14:52

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@MartinSojka: I could finally find the link to the treaty signed on 1951-01-27, added link to the answer. However, the actual border description isn't there - supposedly it spans 584 pages in German (and even more in Polish).
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Wladimir PalantOct 27 '11 at 14:57